To reference an existing scalar field function, the variable name in the new definition will be the existing function name (as defined in the Function Name property of the Expert category) prepended by a dollar ($) symbol.
For example, to reference the field function for temperature, first look at the field function name:
In this case, the name is Temperature. The variable name will thus be
$Temperature
Since the names are case-sensitive, they must be entered in the field function definition exactly as they are shown in the function name.